Delayed-pulse generator having means for stabilizing the charging current

Abstract

A delayed-pulse generator formed by a combination of a comparator, a current-mirror circuit, a capacitor, a switching transistor and a constant-current source further has a transistor whose emitter and base are connected together and which is connected in parallel and in forward-direction with respect to the constant-current source. A charging current of a very small value can be set stably without being affected by the leakage current flowing in the switching transistor even in a high temperature operation. Thus, a pulse having a long delay time can be readily obtained. In addition, an erroneous operation caused by the leakage current flowing in the switching transistor can be effectively suppressed.

What is claimed is: 
     
       1. A delayed-pulse generator comprising: a first and a second power supply terminal;   a comparator circuit having a first input terminal coupled to receive a reference voltage which sets a threshold voltage therefor, said comparator circuit providing a pulse when an input potential at a second input terminal thereof exceeds said reference voltage;   a capacitor having one end connected to said second power supply terminal, the other end of said capacitor being connected to said second input terminal of said comparator circuit;   a current-mirror circuit coupled to output an output current which charges said capacitor;   a first transistor for discharging said capacitor with a predetermined timing;   a constant-current source connected between said current-mirror circuit and said second power supply terminal, for setting said output current supplied from said current-mirror circuit; and   a second transistor connected in parallel and in a forward-direction with respect to said constant-current source said second transistor having a leakage current which is the same as said leakage current of said first transistor so that said capacitor is enabled to be charged by said output current without being affected by a leakage current of said first transistor.   
     
     
       2. The delayed-pulse generator according to claim 1, in which said first and second transistors are NPN-type transistors, said second transistor having a base and an emitter connected in common to said second power supply terminal and having a collector connected to a junction node between said current-mirror circuit and said constant-current source. 
     
     
       3. The delayed-pulse generator according to claim 1, in which a relationship n=m/A is satisfied where m and n respectively represent collector-substrate junction areas of said first and second transistors and A represents the current gain of the current-mirror circuit.